Communication satellite systems have limited bandwidth and power for transmitting downlink communication signals. The amount of effective bandwidth in a communication satellite system directly corresponds to the amount of traffic that can be carried (i.e., the satellite's capacity). Typically, satellite downlink communications are transmitted over a wide area, which may be covered by an array of narrow zone beams or cells to make maximal use of the physical or actual bandwidth that is allocated. This allows the frequency band of the actual bandwidth to be used multiple times and thereby increase the effective bandwidth for satellite communications. Each spot beam is associated with one or more transponders, each of which has its own power requirements. The power requirements of these transponders must be met with the limited power resources of a communication satellite.
One signal degradation problem facing satellite communications, particularly at Ku-band (11-12 GHz) and higher frequency communications, is degradation of the communication signals by weather conditions such as rain. Rain can cause downlink signal attenuation of as much as 20 dB (the higher the rain rate the greater the degradation of the signal from a satellite to a ground recipient). Such extreme attenuation can dramatically degrade recipient signal detection and therefore system availability and capacity. In some places, particularly places with generally equatorial climates, prolonged heavy rains can cause unacceptably prolonged attenuation of satellite downlink communications.
Another communication signal degradation effect is caused by increased signal traffic. When the traffic within a saturated transponder increases (i.e., more signals are added), the power available per signal decreases. Similarly, when traffic takes up more of the available bandwidth of the transponder due to having more signals or signals increasing their bandwidth, the power available per unit bandwidth (i.e., power flux density) decreases. Even with a backed-off or non-saturated transponder, new signals or bandwidth requirement can degrade all signals under some circumstances. The end result is that the recipient's signal detection is degraded or there is a decrease in signal availability.
One way to address such communication signal degradation is to provide a fixed increase in the satellite downlink transmission power for all transponders. There are several problems with this brute force solution. The first is that satellite designs are power limited (e.g., 15-20 kW total power), so power increases might only be limited. Another problem is that such a brute force solution is wasteful. With even prolonged daily peaks in signal traffic load or the prolonged rain seasons of a generally equatorial climate, there are significant amounts of time that do not suffer from these degradation effects. As a consequence, fixed power capabilities for solving these signal degradations are frequently wasted (e.g., during relatively dry periods or periods of low traffic demand).
A variation of the brute force solution is to provide a fixed increase in the satellite downlink transmission power only for transponders corresponding to selected places that suffer from such downlink communication signal degradation. For example, the selected places could correspond to the most densely populated areas to which the satellite transmits. A disadvantage is that this solution also permanently diverts power from other transponders, which power could otherwise be used for periods of high demand and heavy rain. This is a significant problem because, for example, the rain patterns that affect densely populated areas also affect other regions. Relegating these other regions to substandard satellite communication service is a generally unacceptable solution.
U.S. Pat. No. 4,831,619 describes a satellite communication system that provides point-to-point communication by interconnecting multiple terminal sites within an area on the earth. Multiple radio frequency uplink communication signals, each carrying a receive signal, are transmitted from uplink terminal sites in the area to a satellite in geosynchronous orbit above the earth. The uplink signals are received at the satellite and are converted to corresponding transmit signals for transmission to downlink terminal sites in the area.
All of the transmit signals are collectively amplified using an interconnected array of amplifiers on the satellite such that each of the transmit signals is amplified collectively by all of the amplifiers. The satellite transmits multiple downlink signals, wherein each of the downlink signals is directed to a downlink terminal site in a particular portion of the area.
The satellite system of U.S. Pat. No. 4,831,619 provides increased power to downlink signals by increasing the power in corresponding uplink signals. The satellite system operates in conjunction with a ground based network control center that keeps track of all regions in which it is raining. The network control center correlates the downlink regions where it is raining with the corresponding uplink users and instructs each of these uplink users to increase its uplink power for the signals destined for a rain-affected area. The increase in power of the uplink users signals results in greater collective amplification of those signals by the satellite amplifiers.
The satellite system of U.S. Pat. No. 4,831,619 addresses attenuation caused by rain with a ground-based solution that utilizes ground-based increases in communication signal power. While this solution may have applicability to point-to-point communications in which uplink stations are capable of increasing their transmission power, such a solution is poorly suited to broadcast communications in which signals are broadcast over a wide area and to point-to-point communications in which uplink power is limited or fixed. This solution is ill-suited to broadcast communications because they do not have a particular uplink transmitter that is directly associated with a corresponding rain-attenuated downlink receiver. The solution provided for point to point communication described in U.S. Pat. No. 4,831,619 is therefore inapplicable to many communication satellite applications.
The present invention overcomes the deficiencies of prior systems by providing a satellite communications system that dynamically allocates additional downlink resources to selected downlink communication signals to compensate for signal degradation. A typical cause of such degradation is rain where the downlink station is located. Another cause of such degradation is increased communication traffic or bandwidth demand within a particular region.
Dynamic allocation of downlink resources provides increased effective capacity and availability without requiring an increase in the fixed downlink resources on a satellite. Resources are allocated to where they are needed, when they are needed, without having to permanently increase the satellite payload as in the brute force methods. The invention can increase the availability of communication signals even if the transmitting user's uplink power is limited. Also, the invention is applicable to broadcast communications in addition to point-to-point communications.
When all communication signals are transmitted into a region without localized degradation, the signals receive standard amplification to provide good signal strength to all downlink sites. In one embodiment, when degradation is detected at a particular downlink region, the amplification of the signal directed to that region may be increased on the satellite to compensate for the effects of degradation. In this manner, power is allocated efficiently, and downlink signal strength is maintained at strength levels to assure reception.
In one embodiment of signal degradation detection, earth based sensors detect conditions likely to degrade downlink signals (e.g., rainfall at ground downlink stations or increased traffic load). Information about these conditions is conveyed to a ground-based control center, such as by a telephone transmission, where the information is processed to determine if downlink signal resources are to be re-allocated. If the downlink signal resources are to be re-allocated, an appropriate control signal is transmitted to the satellite.
The satellite receives information of the signal degradation and dynamically boosts a signal or signals by increasing amplifier power for the selected signal or signals. The satellite may allocate power among the amplifiers to decrease power to signals corresponding to areas with no signal degradation and increase power to signals corresponding to areas with signal degradation without increasing the total power requirement of the communications system. In another embodiment, a user's signal code rate may be changed in response to information of signal degradation conditions. Another methods of compensating for signal degradation include frequency assignment variation and changing the downlink antenna coverage area.